Chemical structures and compositions of ECP additives to reduce pit defects

ABSTRACT

A composition and method which is suitable to enhance the wetting of an electroplating bath solution on an electroplating surface. Optimum wetting of the electroplating bath solution to the electroplating surface results in an electroplated metal which is substantially devoid of surface pits and other structural defects and is characterized by enhanced gap fill capability. The composition includes a suppressor additive for the electroplating bath solution. The suppressor additive is a copolymer which includes various proportions of ethylene oxide monomer and propylene oxide monomer.

FIELD OF THE INVENTION

The present invention relates to electrochemical plating (ECP) processesused to deposit metal layers on semiconductor wafer substrates in thefabrication of semiconductor integrated circuits. More particularly, thepresent invention relates to a composition and method for enhancingwetting of electrochemical plating electrolyte to a metal seed layer inelectrochemical plating of metals, particularly copper, on a substrate.

BACKGROUND OF THE INVENTION

In the fabrication of semiconductor integrated circuits, metal conductorlines are used to interconnect the multiple components in devicecircuits on a semiconductor wafer. A general process used in thedeposition of metal conductor line patterns on semiconductor wafersincludes deposition of a conducting layer on the silicon wafersubstrate; formation of a photoresist or other mask such as titaniumoxide or silicon oxide, in the form of the desired metal conductor linepattern, using standard lithographic techniques; subjecting the wafersubstrate to a dry etching process to remove the conducting layer fromthe areas not covered by the mask, thereby leaving the metal layer inthe form of the masked conductor line pattern; and removing the masklayer typically using reactive plasma and chlorine gas, thereby exposingthe top surface of the metal conductor lines. Typically, multiplealternating layers of electrically conductive and insulative materialsare sequentially deposited on the wafer substrate, and conductive layersat different levels on the wafer may be electrically connected to eachother by etching vias, or openings, in the insulative layers and fillingthe vias using aluminum, tungsten or other metal to establish electricalconnection between the conductive layers.

Deposition of conductive layers on the wafer substrate can be carriedout using any of a variety of techniques. These include oxidation, LPCVD(low-pressure chemical vapor deposition), APCVD (atmospheric-pressurechemical vapor deposition), and PECVD (plasma-enhanced chemical vapordeposition). In general, chemical vapor deposition involves reactingvapor-phase chemicals that contain the required deposition constituentswith each other to form a nonvolatile film on the wafer substrate.Chemical vapor deposition is the most widely-used method of depositingfilms on wafer substrates in the fabrication of integrated circuits onthe substrates.

Due to the ever-decreasing size of semiconductor components and theever-increasing density of integrated circuits on a wafer, thecomplexity of interconnecting the components in the circuits requiresthat the fabrication processes used to define the metal conductor lineinterconnect patterns be subjected to precise dimensional control.Advances in lithography and masking techniques and dry etchingprocesses, such as RIE (Reactive Ion Etching) and other plasma etchingprocesses, allow production of conducting patterns with widths andspacings in the submicron range. Electrodeposition or electroplating ofmetals on wafer substrates has recently been identified as a promisingtechnique for depositing conductive layers on the substrates in themanufacture of integrated circuits and flat panel displays. Suchelectrodeposition processes have been used to achieve deposition of thecopper or other metal layer with a smooth, level or uniform top surface.Consequently, much effort is currently focused on the design ofelectroplating hardware and chemistry to achieve high-quality films orlayers which are uniform across the entire surface of the substrates andwhich are capable of filling or conforming to very small devicefeatures. Copper has been found to be particularly advantageous as anelectroplating metal.

Electroplated copper provides several advantages over electroplatedaluminum when used in integrated circuit (IC) applications. Copper isless electrically resistive than aluminum and is thus capable of higherfrequencies of operation. Furthermore, copper is more resistant toelectromigration (EM) than is aluminum. This provides an overallenhancement in the reliability of semiconductor devices because circuitswhich have higher current densities and/or lower resistance to EM have atendency to develop voids or open circuits in their metallicinterconnects. These voids or open circuits may cause device failure orburn-in.

A typical standard or conventional electroplating system for depositinga metal such as copper onto a semiconductor wafer includes a standardelectroplating cell having an adjustable current source, a bathcontainer which holds an electrolyte electroplating bath solution(typically acid copper sulfate solution), and a copper anode and acathode immersed in the electrolyte solution. The cathode is thesemiconductor wafer that is to be electroplated with metal. Both theanode and the semiconductor wafer/cathode are connected to the currentsource by means of suitable wiring. The electroplating bath solution mayinclude an additive for filling of submicron features and leveling thesurface of the copper electroplated on the wafer. An electrolyte holdingtank may further be connected to the bath container for the addition ofextra electrolyte solution to the bath container, as needed.

In operation of the electroplating system, the current source applies aselected voltage potential typically at room temperature between theanode and the cathode/wafer. This potential creates a magnetic fieldaround the anode and the cathode/wafer, which magnetic field affects thedistribution of the copper ions in the bath. In a typical copperelectroplating application, a voltage potential of about 2 volts may beapplied for about 2 minutes, and a current of about 4.5 amps flowsbetween the anode and the cathode/wafer. Consequently, copper isoxidized at the anode as electrons from the copper anode and reduce theionic copper in the copper sulfate solution bath to form a copperelectroplate at the interface between the cathode/wafer and the coppersulfate bath.

The copper oxidation reaction which takes place at the anode isillustrated by the following reaction equation:Cu---->Cu⁺⁺+2e⁻

The oxidized copper cation reaction product forms ionic copper sulfatein solution with the sulfate anion in the bath 20:Cu⁺⁺+SO₄ ⁻⁻---->Cu³⁰ ⁺SO₄ ⁻⁻

At the cathode/wafer, the electrons harvested from the anode flowedthrough the wiring reduce copper cations in solution in the coppersulfate bath to electroplate the reduced copper onto the cathode/wafer:Cu⁺⁺+2e⁻---->Cu

When a copper layer is deposited on a substrate, such as byelectrochemical plating, the copper layer must be deposited on a metalseed layer such as copper which is deposited on the substrate prior tothe copper ECP process. Seed layers may be applied to the substrateusing any of a variety of methods, such as by physical vapor deposition(PVD) and chemical vapor deposition (PVD). Typically, metal seed layersare thin (about 50-1500 angstroms thick) in comparison to conductivemetal layers deposited on a semiconductor wafer substrate.

Conventional electrochemical plating techniques use copper sulfate (CuS0₄) for the main electrolyte in the electroplating bath solution. Thesolution may further include additives such as chloride ion andlevelers, as well as accelerators and suppressors which increase anddecrease, respectively, the rate of the electroplating process. The rateof deposition of copper on the substrate, and the quality and resultingelectrical and mechanical properties of the metallization, arecritically dependent on the concentration of these organic additives inthe electroplating bath solution. However, one of the drawbacks of theconventional suppressor used in electroplating bath solutions is thatthe suppressor causes poor wettability of the solution to the copperseed layer on a substrate. Non-uniform wetting of the solution to theseed layer causes structural defects such as pits in the metalelectroplated onto the seed layer, compromising the structural andfunctional integrity of the finished IC devices fabricated on thesubstrate. Further, inadequate wetting of the electrolyte solution togap features results in inadequate filling of the features, particularlywith regard to 65 nm copper technology.

Traditional approaches to improving the wetting of an electrolyteelectroplating bath solution to a metal seed layer include pre-rinsingor pre-annealing of the seed layer surface. However, both of thesemethods achieve unsatisfactory results. Accordingly, a new and improvedcomposition and method is needed for enhancing the wetting of anelectroplating bath solution to a metal seed layer in theelectrochemical plating of copper or other metal on a substrate.

An object of the present invention is to provide a novel composition andmethod for improving the gap fill characteristics and reducing surfacedefects of metal electroplated on a substrate.

Another object of the present invention is to provide a novelcomposition and method for enhancing the wetting of an electroplatingbath solution to a metal seed layer in the electrochemical plating ofcopper or other metal on a substrate.

Still another object of the present invention is to provide a novelcomposition and method which results in electroplating of a metal layersubstantially devoid of structural defects on a seed layer provided on asubstrate and improves gap fill capability through wetting improvement.

Yet another object of the present invention is to provide a novelcomposition which is added to an electroplating bath solution tosubstantially reduce or eliminate the formation of pits or other surfacedefects in an electroplated metal and enhance gap fill capability.

SUMMARY OF THE INVENTION

In accordance with these and other objects and advantages, the presentinvention is generally directed to a novel composition and method whichis suitable to enhance the wetting of an electroplating bath solution onan electroplating surface. Optimum wetting of the electroplating bathsolution to the electroplating surface results in an electroplated metalwhich is substantially devoid of surface pits and other structuraldefects and is characterized by enhanced gap fill capability. Thecomposition includes a suppressor additive for the electroplating bathsolution. The suppressor additive is a copolymer which includes variousproportions of ethylene oxide monomer and propylene oxide monomer.

According to the method of the invention, an electrolyte electroplatingbath solution is prepared, and the suppressor additive copolymer ismixed with the bath solution. The substrate having the electroplatingsurface is immersed in the solution and subjected to electrochemicalplating. During immersion of the substrate, the solution, which ischaracterized by a high capillary rise and low interfacial energy ascompared to electroplating bath solutions having a conventional,commercially-available suppressor additive, rapidly wets theelectroplating surface. This ensures optimal wetting of all regions onthe electroplating surface, including high aspect ratio gap features,and results in uniform electroplating deposition and gap-filling withminimal tendency for immersion-related electroplating defects.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic of an electrochemical plating system inimplementation of the present invention;

FIG. 1A is a cross-sectional view of a substrate with a metal layerelectroplated thereon according to the composition and method of thepresent invention; and

FIG. 2 is a flow diagram illustrating a typical flow of process stepsaccording to the method of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention has particularly beneficial utility in theelectrochemical plating of copper on a copper seed layer deposited on asemiconductor wafer substrate in the fabrication of semiconductorintegrated circuits. However, the invention is more generally applicableto the electrochemical plating of metals including but not limited tocopper on substrates in a variety of industrial applications includingbut not limited to semiconductor fabrication.

The present invention is generally directed to a novel composition andmethod for enhancing the wetting of an electroplating bath solution on aseed layer provided on a substrate. The composition and methodfacilitates the electroplating of a metal which is substantially devoidof surface pits and is characterized by enhanced gap fill capability, onthe seed layer. The composition includes a suppressor additive for theelectroplating bath solution. The suppressor additive is a copolymerwhich includes various proportions of ethylene oxide monomer andpropylene oxide monomer.

In one embodiment, the suppressor additive is a block copolymer ofethylene oxide and propylene oxide. In another embodiment, thesuppressor additive is a random copolymer of ethylene oxide andpropylene oxide. In still another embodiment, the suppressor additive isan alternating copolymer of ethylene oxide and propylene oxide.Preferably, the suppressor additive copolymer comprises at least about60% by weight ethylene oxide. Most preferably, the suppressor additivecopolymer comprises about 80% by weight ethylene oxide and about 20% byweight propylene oxide. The suppressor additive copolymer preferably hasa molecular weight in the range of from typically about 500 to about20,000.

The composition and method of the present invention may be used with anyformulation for the electroplating bath solution, such as copper,aluminum, nickel, chromium, zinc, tin, gold, silver, lead and cadmiumelectroplating baths. The present invention is also suitable for usewith electroplating baths containing mixtures of metals to be platedonto a substrate. It is preferred that the electroplating bath be acopper alloy electroplating bath, and more preferably, a copperelectroplating bath. Typical copper electroplating bath formulations arewell known to those skilled in the art and include, but are not limitedto, an electrolyte and one or more sources of copper ions. Suitableelectrolytes include, but are not limited to, sulfuric acid, aceticacid, fluoroboric acid, methane sulfonic acid, ethane sulfonic acid,trifluormethane sulfonic acid, phenyl sulfonic acid, methyl sulfonicacid, p-toluenesulfonic acid, hydrochloric acid, phosphoric acid and thelike. The acids are typically present in the bath in a concentration inthe range of from about 1 to about 300 g/L. The acids may furtherinclude a source of halide ions such as chloride ions. Suitable sourcesof copper ions include, but are not limited to, copper sulfate, copperchloride, copper acetate, copper nitrate, copper fluoroborate, coppermethane sulfonate, copper phenyl sulfonate and copper p-toluenesulfonate. Such copper ion sources are typically present in aconcentration in the range of from about 10 to about 300 g/L ofelectroplating solution.

In a preferred embodiment, the suppressor additive composition of thepresent invention is present in the electroplating bath solution in aconcentration of from typically about 50 ppm to about 500 ppm. Anaccelerator is typically present in the electrolyte bath solution in aconcentration of from typically about 2 ppm to about 50 ppm. Theaccelerator may be any type of commercially-available accelerator knownby those skilled in the art for accelerating a metal electroplatingdeposition process.

Other electrochemical plating process conditions suitable forimplementation of the present invention include a plating rpm of fromtypically about 0 rpm to about 500 rpm; a plating current of fromtypically about 0.2 mA/cm² to about 20 mA/cm²; and a bath temperature offrom typically about 10 degrees C. to about 35 degrees C. In cases inwhich planarity of the electroplated metal through chemical mechanicalplanarization (CMP) is necessary, a leveling agent may be added to theelectroplating bath solution at a concentration of from typically about5 mmol/L to about 5 mol/L.

Referring to FIG. 1, an electrochemical plating (ECP) system 10 suitablefor implementation of the present invention is shown. The system 10 maybe conventional and includes a standard electroplating cell having anadjustable current source 12, a bath container 14, a typically copperanode 16 and a cathode 18, which cathode 18 is the semiconductor wafersubstrate that is to be electroplated with copper. The anode 16 andcathode/substrate 18 are connected to the current source 12 by means ofsuitable wiring 38. The bath container 14 holds an electrolyteelectroplating bath solution 20. The system 10 may further include amechanism for rotating the substrate 18 in the bath 20 during theelectroplating process, as is known by those skilled in the art.

The ECP system 10 may further include a pair of bypass filter conduits24, a bypass pump/filter 30, and an electrolyte holding tank 34 for theintroduction of additional electrolytes into the bath container 14, asnecessary. The bypass filter conduits 24 typically extend through theanode 16 and open to the upper, oxidizing surface 22 of the anode 16 atopposite ends of the anode 16. The bypass filter conduits 24 connect tothe bypass pump/filter 30 located outside the bath container 14, and thebypass pump/filter 30 is further connected to the electrolyte holdingtank 34 through a tank inlet line 32. The electrolyte holding tank 34is, in turn, connected to the bath container 14 through a tank outletline 36. It is understood that the ECP system 10 heretofore describedrepresents just one example of a possible system which is suitable forimplementation of the present invention, and other systems ofalternative design may be used instead.

Referring to FIGS. 1, 1A and 2, according to the method of the presentinvention, a metal seed layer 19, such as copper, is deposited on awafer substrate 18, as indicated in step S1 of FIG. 2. The metal seedlayer 19 may be deposited on the substrate 18 using conventionalchemical vapor deposition (CVD) or physical vapor deposition (PVD)techniques, according to the knowledge of those skilled in the art. Theseed layer 19 has a thickness of typically about 50-1500 angstroms.

As indicated in step S2 of FIG. 2, the electrochemical plating (ECP)electrolyte bath solution 20 is prepared in the bath container 14. Theelectroplating bath solution 20 typically includes an accelerator havinga concentration of from typically about 5 mmol/L to about 5 mol/L, andmay include a leveling agent or additive in a concentration of fromtypically about 5 mmol/L to about 5 mol/L, as heretofore noted. Next, asindicated in step S3 and shown in FIG. 1, the suppressor additivecopolymer 25 of the present invention is added to and throughly mixedwith the electroplating bath solution 20 to achieve a suppressoradditive concentration of from typically about 5 mmol/L to about 5mol/L. The anode 16 and substrate 18 are then immersed in the bathsolution 20 and connected to the adjustable current source 12 typicallythrough wiring 38.

As indicated in step S4 of FIG. 2, the cathode/substrate 18 is immersedin the bath solution 20. Accordingly, the seed layer 19 on the substrate18 contacts the bath solution 20. The entire surface of the seed layer19, as well as gap features on the substrate 18, are thoroughly wettedby the bath solution 20. It will be appreciated by those skilled in theart that the suppressor additive copolymer composition 25 permitsoptimal wetting of the ECP electrolyte bath solution 20 to the seedlayer 19 during immersion of the substrate 18 and throughout theelectroplating process, as the bath solution 20 lackscommercially-available suppressor additives which have been shown tohinder the wetting capabilities of an electroplating bath solution.

As shown in FIG. 1A and indicated in step S5 of FIG. 2, a metal layer(not shown) is electroplated onto the seed layer 19, typically asfollows. The electroplating bath solution 20 is heated to a temperatureof typically from about 10 degrees C. to about 35 degrees C. Inoperation of the ECP system 10, the current source 12 applies a selectedvoltage potential, typically at room temperature, between the anode 16and the cathode/substrate 18. This voltage potential creates a magneticfield around the anode 16 and the cathode/substrate 18, which magneticfield affects the distribution of the copper ions in the bath solution20. In a typical copper electroplating application, a voltage potentialof about 2 volts may be applied for about 2 minutes, and a platingcurrent of from typically about 0.2 mA/cm² to about 20 mA/cm² flowsbetween the anode 16 and the cathode/substrate 18. The plating rpm forthe substrate 18 is typically about 0-500 rpm. Consequently, copper isoxidized typically at the oxidizing surface 22 of the anode 16 aselectrons from the copper anode 16 reduce the ionic copper in the coppersulfate solution bath 20 to form a copper electroplate (not illustrated)at the interface between the cathode/substrate 18 and the copper sulfatebath 20. Due to thorough and substantially uniform wetting of theelectrolyte bath solution 20 to the entire surface of the seed layer 19,the electroplated metal layer 21 deposited onto the seed layer 19 issubstantially continuous and devoid of structural deformities such aspits. Furthermore, the electroplated metal is particularly effective inhigh aspect ratio gap-filling applications. Accordingly, theelectroplated metal layer 21 on the substrate 18 contributes to thefabrication of high-quality IC devices that are characterized by highstructural and operational integrity.

While the preferred embodiments of the invention have been describedabove, it will be recognized and understood that various modificationscan be made in the invention and the appended claims are intended tocover all such modifications which may fall within the spirit and scopeof the invention.

1. An electrolyte for copper electroplating, comprising: an electrolytesolution; and a copolymer comprising ethylene oxide and propylene oxideproviding in said electrolyte solution.
 2. The electrolyte of claim 1wherein said copolymer is a block copolymer.
 3. The electrolyte of claim1 wherein said ethylene oxide is present in said copolymer in a quantityof at least about 60% by weight.
 4. The electrolyte of claim 1 whereinsaid copolymer is present in said electrolyte solution in aconcentration of from about 50 ppm to about 500 ppm.
 5. The electrolyteof claim 1 wherein said copolymer is a random copolymer.
 6. Theelectrolyte of claim 5 wherein said ethylene oxide is present in saidcopolymer in a quantity of at least about 60% by weight.
 7. Theelectrolyte of claim 1 wherein said copolymer is an alternatingcopolymer.
 8. The electrolyte of claim 7 wherein said ethylene oxide ispresent in said copolymer in a quantity of at least about 60% by weight.9. The electrolyte of claim 1 wherein said ethylene oxide is present insaid copolymer in a quantity of about 80% by weight and said propyleneoxide is present in said copolymer in a quantity of about 20% by weight.10. The electrolyte of claim 9 wherein said copolymer is a blockcopolymer.
 11. The electrolyte of claim 9 wherein said copolymer is arandom copolymer.
 12. The electrolyte of claim 9 wherein said copolymeris an alternating copolymer.
 13. An electrolyte for copperelectroplating, comprising: an electrolyte solution; a copolymercomprising ethylene oxide and propylene oxide providing in saidelectrolyte solution; and a leveling agent provided in said electrolytesolution.
 14. The electrolyte of claim 13 wherein said copolymer is ablock copolymer, a random copolymer or an alternating copolymer.
 15. Theelectrolyte of claim 13 wherein said ethylene oxide is present in saidcopolymer in a quantity of at least about 60% by weight.
 16. Theelectrolyte of claim 13 wherein said copolymer is present in saidelectrolyte solution in a concentration of from about 50 ppm to about500 ppm.
 17. A method of electroplating a metal on an electroplatingsurface, comprising the steps of: providing an electroplating bathsolution; mixing a copolymer comprising ethylene oxide and propyleneoxide with said solution in a concentration of from about 50 ppm toabout 500 ppm; immersing said electroplating surface in said solution;and electroplating said metal onto said electroplating surface.
 18. Themethod of claim 17 wherein said copolymer is a block copolymer, a randomcopolymer or an alternating copolymer.
 19. The method of claim 17wherein said ethylene oxide is present in said copolymer in a quantityof at least about 60% by weight.
 20. The method of claim 17 wherein saidethylene oxide is present in said copolymer in a quantity of about 80%by weight and said propylene oxide is present in said copolymer in aquantity of about 20% by weight.